Use the correct Torx bit, usually T6 or T8, seated fully, and apply firm downward pressure while turning slowly. Most stuck clip screws are held by factory thread locker, and a little heat from a hairdryer softens it. Never use a bit that is slightly too small.
Clip screws are small and almost always carry factory thread locker to stop them backing out in a pocket. That is doing its job; it just makes the first turn hard.
Corrosion is the other cause, particularly on a knife that has been wet.
Seat the largest bit that fits fully into the screw head. Press down hard as you turn, so the bit cannot cam out and round the head. Turn slowly and deliberately rather than snapping at it.
If it will not move, warm the area with a hairdryer for a minute to soften the thread locker, then try again.
Stop. A rounded screw head gets worse with every further attempt. Screw extractors sized for small fasteners exist, and a knife shop can usually get it out.
Order replacement screws before you start if you can: they are model specific and easy to lose. See pocket clip.